There's a strange smell to a haunted house – a stale, musty taste that rises from a ghost's phantom footsteps. It's like every individual has their own particular scent. My mother, God rest her, smelt of lavender washing powder; I had a girlfriend who used a cream that smelt of oranges, so that the aroma rose from her smooth, golden skin. I figure that if a ghost is like a distillation of a human soul, a tangible person rendered down to an invisible, drifting force, the elements of their life might get concentrated in the same way – so the ghost of an outdoor guy smells of woodsmoke and wind on water, a high school jock is hair gel and basketball courts, a car nut is petrol and burning rubber. Like their owners, these scents linger on when they should have faded away – that's why they smell so stale. It's why trained nostrils can pick out a haunted house just by sniffing at the keyhole.

It's why I knew the minute I stepped in that house that I was dealing with no ordinary ghost.

It was Spring, sweet and heady Spring, trees garlanded with translucent green leaves, flowers rising brightly from suburban flowerbeds. The light came through the windows into the front room, spilling in glowing rectangle on the tan kitchen floor and over my battered old shoes. I stood stock still, turning only my head to mark different places. This is where Katie and Micah ate. This is where Micah watched sports with Katie leaning on his shoulder. This is where Katie ate Cookie Dough ice cream on bad days. This is where Micah was murdered.

It could just have been the prickles – that feeling of wrongness you get sometimes when something's a little off kilter. But there was more to it than that. What made the hair rise on the back of my neck (and what had kept the house unsold for a year, although no one would realise) was that beneath the pungency of paint and polish, there was an acrid undertone to the air of the house. The smell you get when a piece of plastic falls on a fire, or a tyre leaves a streak on tarmac. Inside that burning smell were others – gasoline, egg-like sulphur, and something a bit like steak on a barbecue.

This was one heck of a messed up house.

Outside, I heard cautious footsteps coming up the path. I had left the door open (always leave an easy exit), so the owner of the feet involved just leant his head nervously inside.

'You ok, Bobby?'

He always wore a suit, Mark Enesco – I didn't know him as a kid, but I bet he was born with a neck tie. Federal Officers are always so beautifully turned out. They like smart suits, shiny shoes, slicked back hair and clean shaven faces. They like numbers that add up, cases that are solvable, things that make sense. That's why, for all your federal officers, you'll always need bearded, middle aged ranch hands in check shirts and old shoes. They'd never admit as much. But it's always them have to phone me, and not the other way around.

'I'm good, thanks. Nothing much to see here'

He gulped, extending his long neck a little further into the hall but still not quite stepping inside.

'Do you think it's...your kinda thing?'

My kinda thing, indeed. I rolled my eyes at the empty living room, then turned back his way before the poor kid messed his immaculate pants. His relief was obvious as he backed away, warm Spring sunshine lighting up the beads of sweat on his shiny Federal forehead. I stretched, rolled my shoulders, pulled a pack of cigarettes from my pocket and placed one carefully in the corner of my mouth. It's my little ceremony whenever I leave a house – a lot of ghosts don't like smoke and you should keep 'em happy if you can. All the while he was watching me with these big, scared eyes, a little bit pathetic, a little bit hopeful, still a little bit dubious as well. I flicked my cigarette lighter and brought the flame up between us.

'Oh yeah,' I said, smiling with one side of my mouth. 'This is 100% one of mine.'

See, my name is Robert James Carthigan. I'm 42, from Wyoming, drive a good solid pick up truck and watch baseball. I like beer and barbecues. You won't have heard of me: I'm an unextraordinary guy.

Except I chase ghosts. But why would that be of interest to you?